In telecommunication systems, such as cellular communication systems or other radio frequency based communication systems, a mobile subscriber unit must be handed off from a source coverage area to a neighboring coverage area to allow continuous communication for the mobile subscriber unit as it travels across boundaries of coverage areas in a coverage system. There are several types of known handoff arrangements. A hard handoff typically involves communication between the mobile subscriber unit and two base stations such that the mobile subscriber unit only communicates with one base station at a time. A soft handoff typically involves communication by the mobile subscriber unit with multiple base stations at the same time.
Generally, during a soft handoff a source base station communicates with the mobile subscriber unit while the mobile subscriber unit also communicates with a target base station. In code division multiple access (CDMA) telecommunication systems, during a soft handoff, the mobile subscriber unit communicates with the source base station and target base station over a same frequency band. Each base station transmits the same information but spread with independent spreading sequences and receives the mobile subscriber unit transmission for later selection combining at a common point, e.g., the base station controller. However, there are times when it is difficult to set up a soft handoff, for example, when the base stations are controlled by two different and possibly geographically separated base station controllers that may even be connected to different mobile switching centers. The handoff region between base stations controlled by different base station controllers is called a coverage seam. A method for hard handoff across coverage seams is described, for example, in U.S. Pat. No. 5,682,416 assigned to the instant assignee.
A proposed handoff method and system such as that described in international patent application No. PCT/FI95/00389 published Jan. 25, 1996 having an inventor Petri Jolma, discloses neighboring cells along a coverage seam wherein the target cell is served by two independent collocated base stations that are controlled by base station controllers associated with neighboring coverage areas. The operations of the two base stations are independent of each other but their coverage areas and propagation conditions are designed to be substantially identical in an attempt to reduce their mutual interference. In addition, both collocated base stations operate in the same frequency range but employ different spreading codes for their respective channels. The equipment of the two base stations may be implemented by dividing a physical base station into two logical separate sections but which employ the same antenna. Hence the base stations employ the same physical resources (e.g., equipment cabinets, antenna towers and antennas) but the equipment has separate connections for two base station controllers. As a mobile subscriber unit moves across the seam, a source base station carries out a soft handoff to a traffic channel of a base station under the control of the same base station controller. As the mobile subscriber unit continues to move toward the target base station, a hard handoff is performed to switch control from one base station controller to a base station controller controlling the target base station. In such a hard handoff, the spreading code employed by the mobile subscriber unit changes.
Several problems can occur in such a system that utilizes collocated independent base stations to effect a soft handoff across coverage seams. For example with two independent base stations in the same equipment box, each base station typically transmits its own pilot channels for use in providing timing and phase synchronization to aide in subsequent demodulation of a transmitted signal on a traffic channel, as known in the art. Having multiple independent base stations with dual pilot generation, can cause unnecessary interference during handoffs. In addition, where such systems use different spreading codes, from two base stations that are independent of each other, additional noise may be added during handoff operation.
In addition, such a system may not provide adequate integration of its existing hardware and therefore may be more costly. Also, such a system may not provide suitable synchronization between a target base station and the collocated base station. For example, where two independent but collocated base stations transmit from the same antenna, the timing of traffic channels for a handoff may be different, resulting in an inaccurate time alignment and frequency alignment of the soft handoff channels. Also, all the transmitted power of the target base station acts as a noise source to the portion of the mobile subscriber unit receiver trying to demodulate the signal from the collocated base station.
Moreover, it would be desirable to have a soft handoff system that could be implemented regardless of manufacturer differences in base station design. The intermixing of base station equipment of multiple vendors while facilitating soft handoff can improve communication quality and reliability.
Consequently there exists a need for a method and system for facilitating a handoff of a mobile subscriber unit in a telecommunication system having a plurality of base stations that provide code division multiple access communication.